The invention relates to fabricating parts out of compote material with a matrix that is made of ceramic, at least for the most part, which material is referred to herein as CMC.
A field of application of the invention is fabricating structural parts used in the hot portions of turbine engines, in particular aviation turbine engines, for example parts for turbines, after-bodies, or secondary nozzles.
More particularly, the invention relates to fabricating CMC parts having fiber reinforcement made up almost entirely of silicon carbide, i.e. also containing less than 1 atomic percent (at %) of oxygen, and impurities, if any, in the state of traces, which fibers are referred to below SiC fibers, together with an interphase made of boron nitride (BN) that is interposed between the fibers and the matrix.
Such an SiC fiber reinforced part may be fabricated by obtaining a fiber preform, forming a BN interphase coating on the fibers of the preform, and densifying the preform with a matrix that is made of ceramic, at least for the most part.
A fiber preform of shape close to the shape of a part that is to be fabricated may be obtained by shaping a fiber structure, e.g. a structure made by weaving SiC fiber yarns.
A BN interphase coating may be formed by chemical vapor infiltration (CVI), the preform being held in a desired shape by means of tooling or a shaper.
One such method of fabricating a CMC part is described in Document WO 98/23555. The BN interphase coating is formed by CVI on the SiC fibers from a reaction gas comprising boron trichloride BCl3, ammonia NH3, and gaseous hydrogen H2. The CVI process is performed at a relatively low temperature of 700° C. under a relatively low pressure of 1.3 kilopascals (kPa), so as to obtain a BN interphase providing relatively strong bonding between the fibers and the interphase coating. Such a strong bond makes it possible to take advantage of the capacity of SiC fiber yarns for elastic deformation to obtain a CMC having a high elastic deformation limit, and thus being less susceptible to cracking under load.
Nevertheless, a BN interphase coating obtained under the above conditions is sensitive to oxidation and to moisture, which can lead to it being degraded after being exposed to an oxidizing or corrosive environment, thereby affecting the mechanical properties of the CMC.
It is known that a BN deposit obtained by CVI or by chemical vapor deposition (CVD) presents ability to withstand oxidation that can be improved by imparting a high degree of crystallization to the BN. That can be obtained by performing CVI or CVD deposition at a temperature that is “high”, typically higher than 1300° C., or by subjecting a BN deposit that has been obtained by low temperature CVI to heat treatment at a higher temperature, typically higher than 1300° C.
When depositing a BN interphase by CVI on the fibers of a fiber preform, performing the CVI process at high temperature leads to a thickness gradient for the interphase, which gradient is more marked when the preform is thicker. BN deposition occurs preferentially in the vicinity of the outer surface of the preform with the reaction gas becoming depleted rapidly on diffusing into the core of the preform, thereby leading to thickness that is much smaller in the core of the preform than in the vicinity of its outer surface.
In order to avoid such an interphase thickness gradient, a first solution consists in depositing the BN interphase by CVI or CVD at high temperature on the SiC fiber yarns before making the preform. Nevertheless, the thickness of the interphase must then be small in order to conserve sufficient flexibility for the yarns to enable them to be subjected to textile operations such as weaving, and there is a high risk of such a thin interphase being damaged during such textile operations.
A second solution consists in depositing the BN interphase by CVI at “low” temperature, as in WO 98/23555, and in performing subsequent heat treatment. Nevertheless, and as explained in the publication by S. LeGallet et al. entitled “Microstructural and microtextual investigations of boron nitride deposited from BCl3—NH3—H2 gas mixtures” (Journal of the European Ceramic Society 24 (2004), 33-44), it is possible to increase the degree of crystallization of a BN deposit by heat treatment only if the deposit contains oxygen, e.g. because it has been exposed to an oxidizing atmosphere before the heat treatment. Unfortunately, the presence of oxygen in a BN interphase of a CMC material raises problems, in particular because of the risk of reaction with the SiC fiber at high temperature and to the production of potentially undesirable volatile species (in particular SiO).